This invention relates to a zinc-nickel alloy plating solution, particularly to a zinc-nickel alloy plating solution with good brightness and excellent corrosion resistance.
Zinc-nickel alloy plating has extremely excellent corrosion resistance as compared with zinc plating, being particularly excellent in corrosion resistance under heated environment of around 200.degree. C. and therefore it is attracting attention and some of its plating solution compositions have been proposed.
Uses of zinc type alloy plating may be broadly classified into production of plated steel plates and provision of general parts (press work articles, bolts, etc.). Plated steel plates are generally produced by high speed plating (30 to 100 A/dm.sup.2) from sulfuric acidic bath, and zinc-iron alloy plating making great account of coating adhesion or zinc-nickel alloy plating excellent in corrosion resistance, etc., has been known (e.g. Japanese Laid-open Patent Publication No. 152194/1980). However, these platings have been developed as the plating of steel plates for base plates to be coated, and are not suitable for plating of general parts, because they are poor in brightness and also plating cannot be attached onto the low current density portion (concave portion).
Zinc type alloy plating of general parts is attracting attention for making corrosion resistance of zinc plating higher as a counter measure against injury of salt of automobile parts, and already widely prevailing zinc-iron alloy plating based on zincate bath zinc plating (Japanese Laid-open Patent Publication No. 181293/1985) or zinc-nickel alloy plating based on chloride bath zinc plating (Japanese Laid-open Patent Publication No. 34189/1983) has been known.
Zinc-iron alloy plating from a zincate bath, while having much advantages such as uniform alloy composition obtained, excellent corrosion resistance, low cost, easy chromate treatment, etc., has the drawback that corrosion resistance is abruptly worsened when heat of 150.degree. C. or higher is applied, and therefore not suitable for parts subjected to heat. On the other hand, zinc-nickel alloy plating, while having the advantage of maintenance of excellent corrosion resistance without deterioration of corrosion resistance even when subjected to heat, involves the drawbacks that corrosion resistance is not stable due to great difference in nickel co-deposition amount depending on current distribution and also that the cost is higher due to higher nickel ion concentration in the bath, and therefore other baths are demanded. For such reasons, it has been also attempted to obtain a uniform alloy plating coating of high corrosion resistance at low cost by applying the zincate bath for zinc-nickel alloy plating, but as the result no satisfactory brightness could be obtained as plating of general parts.
That is, it has been proposed to apply zincate bath for zinc-nickel alloy plating ("Investigations of Complexing Agents for Alkaline Zn-Ni Alloy Plating Bath", Gists of 70th Academic Lecture Meeting of Society of Metal Surface Technology). According to this proposal, by use of an appropriate complexing agent (Ni solubilizing agent), a zincate bath lowered in both concentrations of zinc and nickel can be constituted, whereby the production cost can be lowered. As effective complexing agents, tartaric acid, gluconic acid, ethylenediamine and derivatives thereof are reported. Whereas, the zinc-nickel alloy plating thus obtained has no brightness, and the Ni co-precipitation ratio at the low current density portion becomes 2-fold or more of that at medium to high current density portion, as unsuitable for chromate treatment for improvement of corrosion resistance and appearance.
In the prior art, a large number of brighteners for giving brightness to plated film have been proposed. The present applicant attempted to add most of these known brighteners into alkaline zinc-nickel alloy zincate bath, only to obtain substantially no effect. Particularly, there was no satisfactory brightness at the low current density portion which is important to plating at the concave portion of general parts.